1. Field of the Invention
The present invention relates to an ultrasonic-wave propagation-time measuring method and to a gas concentration sensor.
2. Description of the Related Art
Techniques for measuring sound velocity on the basis of propagation time of ultrasonic waves have been proposed in, for example, Japanese Patent Publication (kokoku) No. 5-72527 and Japanese Kohyo (PCT) Patent Publication No. 60-502171.
In these techniques, an ultrasonic element is caused to transmit an ultrasonic wave (transmission wave) and receive its reflection wave (reception wave); and sound velocity is measured on the basis of the propagation time between transmission of the transmission wave and reception of the reception wave.
In a well-known method of measuring propagation time, more particularly, with regard to detecting the reception signal, a threshold level (reference value) of a comparator is fixed, and the reception wave itself or an integral value of the reception wave is compared with the fixed threshold level.
However, ultrasonic reception waves sometimes attenuate due to pressure or other causes. In such a case, the conventional techniques use a fixed value for the threshold level of the comparator, a measured propagation time includes an error stemming from attenuation of the reception wave.
Thus, in some cases a sound velocity calculated from the propagation time may become inaccurate.